With the widespread availability of the Internet in connection with hearing-adapted audio encoding methods, a simple worldwide distribution of high-quality audio signals has become possible. In particular, this has lead to a worldwide wave of music piracy, wherein people for example encode purchased CD music according to the standard MPEG layer-3 (MP3) and place it illegally on the World Wide Web (WWW). According to estimates, about 10 million downloads of music is exchanged in a day, without the holders of the corresponding copies and license rights having authorised same or having received the respective payments therefore. This has lead to great concern in the music industry.
Nowadays, there are, in particular, many constrains concerning the situation of music distribution. Firstly, a wide spread know-how concerning the audio compression technology exists, manifested, for example, in the standard MPEG layer-3 (MP3). Additionally, software encoders, software decoders and MP3-players (for example, mp3enc, 13enc, WinPlay3) and other formats run on a number of operating systems, including the Windows operating system. Furthermore, many Internet locations exist offering MP3-music that has often been placed there without authorisation.
Apart from the software encoders and decoders, hardware players exist as well, for example, MPLayer3, MP-Man, Rio, etc., that are able to play MP3 pieces that have either been encoded from a CD or are files that have been downloaded from the Internet. These players have, so far, no implemented protection techniques for enforcing copy or license rights. Additionally, devices for writing to CD-ROMs exist that are able to write to audio CDs and MP3 CD-ROMs. Meanwhile, these devices are offered to prices that have lead to a wide spread availability. Furthermore, the prices for high-volume hard discs have fallen, which is why most Internet participants have almost unlimited memory capacities. Finally, we want to point out the tendency that transmission costs for files keep falling.
While in the above hardware players no protection techniques have been implemented, there are still several techniques for protecting audio and/or video data (i.e. multimedia data, naming, for example, the multimedia protection protocol MMP. This technology represents a so-called “Secure Envelope” technique.
DE 196 25 625 C1 describes such a technique for encrypting and decrypting of multimedia data. Data encoded according to an audio or video standard are encrypted at least partly via, for example, a DES encryption method (DES=Data Encryption Standard) and written into a payload data block.
The payload data block is provided with a determination data block comprising, apart from a plurality of further information, also information concerning the encryption algorithm used with the encryption as well as the key needed therefore. The key comprises user information such that only a specific user who is authorised for playing a piece of multimedia, for example, by purchase or licensing can decrypt the piece. A player that does not have the correct key will stop operating as soon as it encounters the encrypted multimedia data. Thereby, the objective that only the authorised user can play a piece of multimedia is achieved. This Secure Envelope technique therefore represents a two level method wherein a piece of multimedia is encoded first in order to obtain a significant data compression and wherein then a cryptographical algorithm is used in order to defend the encoded piece of multimedia against unauthorised attackers.
For applications that do not require such maximum protection, the described concept is disadvantageously in that it can become relatively expensive and can require significant modifications to players in order to be able to process the determination data block. The players that are mass products in the consumer area after all, and therefore have to be offered inexpensively should, however, if possible, not having to be changed at all in order to be able to play protected pieces of multimedia. Thus, it has to be noted that the known encryption concept makes a maximum protection and a high encryption flexibility possible by respectively designing the start block, but that, however, distinctive changes with the players are necessary in order to decrypt encrypted files or to read them at all.
U.S. Pat. No. 5,796,838 discloses a method and an apparatus for carrying out an inversion of a frequency spectrum. The inversion of the frequency spectrum is achieved by converting a non-encrypted audio signal from analog to digital. The audio signal is then subjected to a positive complex frequency translation such that the negative frequency components of the audio signal will be positioned at around 0 Hz. The audio signal converted regarding to its frequency will then be low-pass filtered, so that only the base band components are left. The filtered complex base band signal will then be subjected to an arbitrary complex frequency displacement in order to position the signal frequency in a desired frequency band. The resulting signal has an inverted spectrum relative to the original audio signal. Extracting the real part of the complex samples will generate the final audio signal.
U.S. Pat. No. 4,534,037 discloses a method and an apparatus for a scrambled pulse code modulation transmission or recording. In order to emphasise special spectral components of a sequence of digital signals that have been transmitted or recorded in pulse code words, “repacked” words are established comprising one or several bits of a word of the original sequence and a complementary number of bits of the following word. The bits of a word out of which a “repacked” word consists of will be inverted prior to repacking such that, for example, the fourfold sampling frequency, the double sampling frequency or the sampling frequency itself can be emphasised in the spectrum. It is possible to transform the frequency spectrum downward regarding to the frequency without inversion of words.